1. Field of the Invention
The present invention relates to a method of reducing sulfur dioxide emissions of a circulating fluidized bed (CFB) boiler by incorporating a sulfur-reduction stage in the flue gas path.
2. Description of Related Art
Carbonaceous fuel, such as coal, is combusted in the furnace of a CFB boiler in a bed comprising at least one generally inert material, such as sand, and a sulfur dioxide-reducing additive, such as limestone. A fluidizing gas, usually air, is introduced through a bottom grid of the reactor to fluidize the bed material and to oxidize the fuel. Meanwhile, sulfur in the fuel oxidizes mainly to form sulfur dioxide (SO2), which may be harmful if emitted to the environment in large quantities. At the high temperatures prevailing in the furnace, usually from 750° C. to 900° C., calcium carbonate (CaCO3) of the limestone is calcined to form calcium oxide (CaO), which converts the SO2 to calcium sulfate (CaSO4), which can be removed from the furnace along with the ashes produced in the combustion.
Although a relatively good sulfur-reduction efficiency can be obtained in CFB boilers solely by feeding a sulfur dioxide-reducing additive, usually limestone (calcium carbonate), directly into the furnace, in order to achieve 98% or better reduction efficiency in the furnace, the reducing additive has to be fed into the furnace in abundance to the sulfur in the fuel. For example, whereas limestone often is added at a rate providing a Ca/S molar ratio of at least 1.5 to 3, in order to achieve a very high reduction efficiency of above 98%, Ca/S ratios as high as 4 to 5 are required. With such high Ca/S ratios, the bottom ash and fly ash discharged from the furnace invariably contain a large amount of excess CaO, typically more than 20%, which makes the use or disposal of the ashes difficult.
Another problem associated with the conventional sulfur-reduction process in a CFB furnace is that the calcination of calcium carbonate is an endothermic reaction, with a reaction energy of 178.4 kJ/kmol. Thus, the calcination of excessive amounts of limestone to form calcium oxide decreases the thermal efficiency of the boiler. For example, in order to achieve 98% sulfur reduction when combusting coal containing 2% sulfur, limestone is introduced at a rate providing a Ca/S ratio of 5, and the energy required for calcination reduces the thermal efficiency of the boiler by about 2 percentage points.
U.S. Pat. No. 4,309,393 discloses a sulfur-reduction method for a fluidized bed boiler, wherein limestone is added to the furnace in Ca/S ratios ranging from 1 to 1.5, so as to provide sulfur reduction of 30 to 60% in the furnace. The ashes produced in the furnace, which contain a considerable amount of CaO, are collected and treated for utilization in another sulfur-reduction stage disposed in the flue gas duct downstream of the reactor.